Plasmids have been identified in various strains of Streptococcus mutans and Lactobacillus casei. A new lysis technique for attaining much higher yields of DNA has been developed. Work is directed at determining the size and relatedness of these extrachromosomal elements via electron microscopy and DNA-DNA hybridization. Further studies should allow the determination of the phenotypic properties coded for by these plasmids. Of particular interest will be to determine if these elements have played a role in the recent adaptation of microbes to the oral environment and have implanted some part of the pathogenic potential of these organisms.